Hydrocyclones are compact, centrifugal separators with no moving parts, which separate liquids in a liquid mixture. Hydrocyclones are widely used in both onshore and offshore oil production in above-ground applications such as bulk water knockout from produced fluids, de-oiling produced water prior to either water reinjection into a formation or water disposal to a disposal site. In these applications a plurality of hydrocyclones are typically mounted within a pressure vessel assembly. Such an assembly resembles a shell-and-tube heat exchanger, in that the hydrocyclones are mounted to tube sheets which are sandwiched between flanges in the pressure vessel. The complete pressure vessel assembly typically has a single inlet for the produced liquid stream, which comprises as for example, a mixture of oil and water and a plurality of outlets for the separated liquid streams. The assembly has an outlet for the "clean water" stream, which is relatively depleted in oil as compared to the production liquids, and an outlet for the "dry oil" stream, which is relatively enriched in oil as compared to the produced liquids.
Hydrocyclones, as they are employed in oil production and environmental cleanup applications are designed foremost to remove oil from water, that is, to produce a clean water stream with as low a concentration of oil as practicable. The dry oil stream will typically contain about 50 percent water, by volume, and may contain more than 50 percent water. Hydrocyclones, in a single-stage configuration, cannot produce both a completely water-free oil stream and a completely oil-free water stream; the design performance must be biased towards either the "dry oil" stream or the "clean water" stream. A clean water stream is obtained at the expense of "wet oil". Conversely, a dry oil stream is obtained at the expense of oily water. Hydrocyclone designs that are exemplary of those in the art are described in British Patent Application GB-A-2248198, and U.S. Pat. No. 4,237,006, which is incorporated herein by reference for all purposes. Multi-stage separator assemblies including multiple hydrocyclones arranged in series, such as taught by U.S. Pat. No. 4,738,779, incorporated herein by reference for all purposes, can achieve improved separation at the expense of increasing the pressure drop of the liquids moving through the multi-stage assembly.
Hydrocyclones are also useful for making a preliminary separation of oil from water in the production liquids produced downhole in an oil well prior to the production liquids being transported to the surface. This is of particular value in high water cut wells, with a high water content, where the production liquids may comprise about 70 percent, or more, water. Conventionally, this water must be transported above ground, at significant cost and then disposed of, at additional expense. Hydrocyclone assemblies designed for above-ground use however, are not suitable for downhole applications where the assembly must be disposed within the bore hole of an oil well. This is because conventional hydrocyclone assemblies of sufficient capacity exceed the size limitations imposed by the diameter of the well. Further, previous attempts to overcome these problems have resulted in additional complications.
For example, PCT International Application WO 94/13930 discloses a downhole separation apparatus in which one or more hydrocyclones are contained within an axially elongate tubular housing, with the inlet of each hydrocyclone extending through the wall of the housing and having an opening external of the housing. The separated dry oil and clean water streams from each hydrocyclone are transported from the housing by a relatively complex system of pipes. With this apparatus there must be sufficient clearance between the housing and the adjacent wall of the well casing to provide a flow annulus for transporting the production fluid to the hydrocyclone inlets. This limits the diameter of the hydrocyclone housing for a given size casing, and hence reduces the capacity of the separation apparatus. Further, the internal space within the housing, but outside of the separators and piping, is dry, so that there is a very substantial pressure differential across the walls both of the housing and the piping within the housing. Further, the housing must be tightly sealed against the full well bore pressure. This obviously requires the use of heavy gauge and/or specialty materials for construction of the housing, which results in increased costs for both materials and fabrication, and increases the risk of failure of the assembly.
In applications where the pressure of the liquids in the well bore is too low, pumps and associated pump driving equipment, are required. WO 94/13930 for example, discloses placing a pump on the clean water stream to assist in reinjection of the clean water into the formation. This does not address the important problem of transporting the dry oil stream to the surface however. U.S. Pat. No. 5,296,153 discloses pumping the dry oil stream to the surface and the clean water stream to another formation. This further increases the cost and complexity of oil production, exacerbates the problem of locating the equipment within the well bore, and requires pumping the clean water stream, which increases both the capital and operating costs of oil recovery.
The present invention overcomes the deficiencies of the prior art.